With virtually all electrochemical cells, separator members must be interposed between adjacent electrodes of opposite polarities to prevent electronic conduction (shorting) which could result from either direct or dendritic contact. Additionally, the separator members must enable the free flow of ion (ionic conductivity), the flow of solvent molecules (mass transport), and the flow of gas molecules (gas transport). In addition to these functions, the separators may be required to provide surface support for the adjacent electrodes (containment) and storage of electrolyte within its pores. Unfortunately, the simultaneous requirements for containment and electrolyte storage tend to be mutually exclusive, since effective containment requires axial pressure which, in turn, leads to separator compression and loss of porosity.
With flooded systems (separators electrolyte saturated), gases produced at electrode surfaces must be transported to the periphery of the electrodes and expelled. In general, this requires that gas transport channels be included in the separator. The inclusion of such channels generally compromises the containment potential of the separator which, in turn, leads to reduced life.
With starved designs (separators not saturated with electrolyte), gases produced at one electrode are generally consumed at the opposing electrode and, therefore, only axial gas transport is required. However, with most starved systems, a defined, controlled amount of electrolyte storage is required. Since conventional separators are incapable of simultaneously achieving high porosity and low compliance, the simultaneous requirements for good containment (axial pressure), electrolyte storage and high porosity are not achievable. Because of this dilemma, the usual solution is to compromise the containment function of the separator. This, in turn, leads to reduced life and non-economical design. Tubular and spiral battery configurations have also utilized separators for containment of electrolyte. They suffer from high cost and low performance.
In order to remedy the above problems, a separator system is desired having the following simultaneous properties and capabilities:
1. Effective dendrite barrier. PA1 2. Good ionic conductivity. PA1 3. Good mass transport characteristics. PA1 4. Accurately defined thickness. PA1 5. High stiffness constant (low compliance). PA1 6. Uniform surfaces having small pore sizes. PA1 7. Lateral gas transport capability for flooded designs. PA1 8. Axial gas transport for starved designs.